utils/scanpypi: add missing setuptools fallback

Some projects use pyproject.toml without setting a build backend, per
pep517 we should simply fallback to using setuptools when no build
backend is specified.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

utils/scanpypi

@@ -424,6 +424,9 @@ class BuildrootPackage():
                         self.setup_metadata['method'] = 'pep517'
                     else:
                         self.setup_metadata['method'] = 'unknown'
+            else:
+                self.build_backend = 'setuptools.build_meta'
+                self.setup_metadata = {'method': 'setuptools'}
         except FileNotFoundError:
             self.build_backend = 'setuptools.build_meta'
             self.setup_metadata = {'method': 'setuptools'}